In the market of electric wheel set, the frame of the electrical power assist vehicle is connected to at least one wheel, and in the center of each wheel there is a wheel hub, which is fastened to the frame using bolts. For the convenience of delivery, the electric wheel set normally can be removed from the frame of the electrical power assist vehicle. However, it is not easy to remove the electric wheel set from the frame of the electrical power assist vehicle and in most cases the wheel and the power module need to be removed together. Once the electric wheel set is removed from the electrical power assist vehicle, the electrical vehicle, having no power, cannot function normally and the removing process needs to be assisted by extra operator. Besides, even if the power module can be removed alone, many alignment problems will arise during installation. For example, due to the influence between the stop mechanism, the gear mechanism, and the clutch mechanism, the installation of the power module requires many times of angle adjustment, and therefore greatly decreases the user's willingness for installation and jeopardizes the convenience of installation.